With the removal of lead additives, e.g., tetraethyl lead and tetramethyl lead, from gasoline in order to reduce air pollution, it has been found that the lead had acted not only as an antiknock agent, but was also effective in contributing towards the prevention of valve seat recession. In the conventional internal combustion gasoline engine, the inlet and exhaust valves generally seat against their valve seats with a slight rotary motion. This rotary motion is imparted to the valve stem during its operation to constantly shift the relative position of the valve to prevent uneven wear on the valve tip. This rotary motion also prevents the valve from seating exactly the same every operation. With the elimination of the lead additives from gasoline, it has been found that a drastic increase in wear of the valve seat occurs. In fact, actual recession of the valve seat occurs, probably partly due, or aggravated, by rubbing of the valve against its seat as the aforesaid rotary motion occurs. The result is an eventual loss of compression and power.
Recession or exhaust-valve seat wear apparently results from a wearing of the valve seat by the exhaust valve. The seat material is slowly worn away, while the harder valve escapes relatively undamaged. It is believed that lead prevents this problem by forming protective oxide layers.
Valve seat wear is a function of engine design, load and speed conditions, and valve operating temperature. Valve seat wear is most severe under high speed, high load conditions with rotating valves. Recession may still occur in engines not equipped with valve rotators and operating under stop-and-go conditions.
In the past, sodium additives have been suggested for lubricating oils. For example, U.S. Pat. No. 3,182,019 discloses the use of sodium containing materials as oil additives, and U.S. Pat. No. 2,616,904 discloses the use of alkaline earth metal containing materials as oil additives. The addition of sodium additives to lubricating oils, however, is usually made only in premium high cost motor oils, and, thus, engine users selecting a lower grade oil will not derive the benefits to be obtained from the use of sodium additives in lubricating oils. Further, although the use of sodium additives in lubricating oils reduces valve seat wear, it does not eliminate all recession that may occur under severe driving conditions.